you said this sound started right after having the tires rotated?
it could just be the wear on the tires.
if the tread is a little uneven the tires will sometimes make a moaning sound at certain speeds which i would guess is because the tire is burning down the uneven threads. if you can not find the source of the problem i would suggest rotating the tires from front to back just to see if the noise moves. but that's just a wild guess

Last year my '89 420 SEL started making howling and screeching noises in the back that brake application stopped. I finally ran it down to rust build-up in the inner drum of the rotor causing contact. I put on new rotors and e-brake shoes and haven't heard a thing since.